In this specification, for convenience, the term "grain" will be used in the sense that it encompasses not only grain but other particulate foodstuffs or commodities that are commonly stored in bulk, such as peanuts, lentils, peas and other pulses. This list is not intended to be exhaustive. Also, in this specification, the term "grain pests" will encompass the pests usually found in stored "grain" and well known to persons who work in grain storages, being predominantly beetles and some species of moths.
For many years, various chemical pesticides have been applied to stored grain to kill the grain pests that may be present. Many of such chemical pesticides leave residues which can be harmful, and care has to be exercised to ensure that the relevant maximum residue limit is not exceeded. In addition, as noted in the specification of U.S. Pat. No. 3,614,841 to G W Query, there have been "industry . . . accepted practices which are directed to periodic overdoses of insecticide and result in wastage of food products in hopes that such periodic overkill would provide . . . residual protection . . . before the infestation of the food product became sufficiently advanced to require another drastic treatment".
To avoid this last-mentioned problem, Query treats growing crops in greenhouses and the exterior surfaces of multiple storage bins located within essentially sealed enclosures by periodically administering a predetermined quantity of insecticide through spray nozzles in overhead conduits which are located within the greenhouses or enclosures. To ensure that the appropriate quantity of insecticide is used, Query has suggested two techniques. One of those techniques is the discharge into an enclosure of the entire contents of a single canister containing a liquid insecticide and an aerosol propellant, by puncturing the canister with a "puncturing opener" which is connected to a flexible hose that leads to the conduit which contains the overhead spray nozzles. Query's second technique requires periodic injection, into the overhead conduit of each greenhouse or bin enclosure, of a pre-measured quantity of a liquid insecticide, which has been stored with a propellant gas in a respective container mounted outside each greenhouse or bin enclosure. After each administration of insecticide, the respective external container is re-charged with a mixture of the liquid insecticide and propellant gas from a source of such a mixture.
The aforementioned problem with residues has led to the preference for using gaseous fumigants instead of the pesticidal chemicals. And among the gaseous fumigants, phosphine has been preferred because any residue that might be left in the grain will be lost or oxidized to harmless phosphate when the grain is processed to produce a food.
The main problem with applying a gaseous fumigant such as phosphine to a grain storage (such as a silo) concerns dosage rates and maintaining an environment within the grain which ensures proper elimination of the pests. As noted by the present inventor in his paper entitled "Flow-through phosphine fumigation--a new technique" which was delivered to the Stored Grain Protection Conference, 1983, if the so-called "one-shot" or "one-pass" technique is used with phosphine from a gas source (for example, from the reaction of moisture with a solid formulation containing aluminium phosphide) being applied to a leaking silo, the concentration of phosphine within the grain in the silo is likely to decay to zero in about 4 or 5 days. Thus the fumigant is ineffective after about 5 days. Even when a fumigant is applied to grain in a completely sealed silo (for example using the techniques described in the specification of U.S. Pat. No. 4,200,657 to J S Cook), there is a decay of the effective concentration of phosphine in the grain as it is absorbed by the grain and used to exterminate grain pests. Thus, whether the fumigant is applied to the grain utilising Cook's one-pass technique or his recirculating technique (each technique, Cook claims, results in a uniform distribution of phosphine or other fumigant within the grain mass before the forced gas flow through the grain mass is discontinued), there will be a significant fall-off in effectiveness of the technique after a relatively short time period. A similar significant reduction of effectiveness is experienced with the one-shot fumigation technique described in the specification of South African patent No 86/4806, which corresponds to Australian patent No 589,646, in the name of The Commonwealth Industrial Gases Limited. That technique simply requires the release of a phosphine-containing gas into the grain mass stored within a silo.
For complete elimination of grain pests, it is essential that a sufficiently high dosage of phosphine remains in the grain mass long enough to ensure that the more tolerant stages (eggs and pupae) in the development of an insect pest mature into a less tolerant stage (larvae or adults) and are killed by the phosphine. In this way, resistant strains of the pests cannot develop.
In a substantially gas-tight silo, the phosphine concentration decays to zero in about 16 days. The work required to make old grain storages gas-tight is costly and is not always successful. An improved fumigation technique, which ensures that an adequate phosphine dosage is always achieved within the grain, is the subject of Australian patent No 640,669.
In many grain storage establishments, a number of silos of different shapes and sizes are built in close proximity to each other and grain is deposited into an empty silo or removed from a silo containing grain, as required. In such systems, as in any multi-silo storage facility, it would be advantageous to provide a single fumigation arrangement, utilizing a single source of phosphine (or other suitable gaseous fumigant) and carrier gas, which ensures that any number of the silos in the facility can be fumigated simultaneously, using fumigant from the single source of gaseous fumigant, without the need to re-design the fumigation system each time a silo is brought into use for grain storage, or is removed from the system because it is about to be emptied of grain.